The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the acquisition of three-dimensional NMR data from which images may be reconstructed.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins, and after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The present invention will be described in detail with reference to a variant of the well known Fourier transform (FT) imaging technique, which is frequently referred to as "spin-warp". The spin-warp technique is discussed in an article entitled "Spin Warp NMR Imaging and Applications to Human Whole-Body Imaging" by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NXR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (G.sub.y) along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient (G.sub.x) in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse G.sub.y is incremented (.DELTA.G.sub.y) in the sequence of views that are acquired during the scan to produce a set of NMR data from which an entire image can be reconstructed.
The use of three-dimensional versions of the spin-warp method (3DFT) is finding wider use in clinical applications. In the 3DFT implementation spatial information is encoded in two directions by applying phase encoding gradients along both directions and acquiring the NMR signal in the presence of a readout gradient along the third direction. In a typical 3DFT scan, one of the phase encoding gradients (eg. G.sub.z) is stepped through all its values, and for each G.sub.z step, the other phase encoding gradient (eg. G.sub.y) is stepped through all its values. Such a scan is depicted in FIG. 2 where it can be observed that the sampling starts in one corner of "k-space" and finishes in the opposite corner.
In a number of 3DFT clinical applications it is important to acquire the NMR data at a critical moment. For example, in dynamic studies a contrast agent is injected and image contrast is enhanced if the important phase encoding views are acquired when the contrast agent passes through the vasculature of interest. Similarly, preparation techniques such as inversion recovery sequences for suppressing background signal rely on the timing of the NMR data acquisition. Since 3DFT scans require considerable time to acquire all the phase encoding views needed for an image reconstruction, methods must be used in which some of the NMR data is acquired under less than ideal circumstances.
The centric view ordering described in U.S. Pat. No. 5,122,747 is the solution to this timing problem. As illustrated in FIG. 3, centric view ordering scans k-space in a spiral pattern starting at the center of k-space and working outward. Since the central views contain the majority of structural information about the object, these central views are acquired at the optimal moment during the procedure and the peripheral views are acquired later. In FIG. 3, equal y-axis and z-axis fields of view are assumed and hence the spacings .DELTA.k.sub.y and .DELTA.k.sub.z between samples are equal. Because of this equal spacing, the sampling path is a square spiral from which this phase encoding scheme takes its name.
In most clinical applications the fields of view along the two phase encoding axes are far from the same. Typically, the field of view along one axis may be on the order of eight times the field of view along the other axis. This situation is illustrated in FIG. 4 where the field of view is eight times smaller in the z direction than in the y direction, representing, for example, a 256.times.32 (y.times.z) acquisition with equal y and z resolution. The circle encloses the most central 16 phase encoding views as determined by k-space magnitude. It is these views which contribute most to the reconstructed image, and it is these views that should be acquired first. However, with the spiral scan the order in which the views are acquired (indicated by numbers next to each k-space sample point) give too high weighting to sample points along the smaller field of view direction and too low weighting to those along the larger field of view direction. For example, the k-space sample points acquired 60th and 61st in the spiral scan lie much closer to the k-space origin than the k-space samples acquired 5th and 6th during the scan. The improved image quality produced by centric view ordering is thus diminished considerably when unequal fields of view are acquired in the phase encoding directions.